Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Electrochemical amperometric sensors measure concentrations of an analyte by measuring currents caused by oxidation or reduction reactions of the analyte in the presence of a charged electrode. Generally, a negatively charged electrode donates electrons to the analyte in a reduction reaction where the analyte becomes more negatively charged, whereas a positively charged electrode receives electrons from the analyte in an oxidation (or ionization) reaction where the analyte becomes more positively charged. The charged electrode (or working electrode) induces the reactions and receives or donates electrons to generate a current that provides the output signal. Thus, when the working electrode is appropriately charged, the output current is proportional to the reaction rate, which provides a measure of the concentration of the analyte surrounding the working electrode.
In some examples, an enzyme is fixed proximate the working electrode to selectively react with a desired analyte. For example, glucose oxidase can be fixed near the working electrode to react with glucose and release hydrogen peroxide, which is then electrochemically detected by the working electrode to indicate the presence of glucose. Other enzymes and/or reagents can be used to detect other analytes.
The eye is coated by a layer of tear film secreted by the lacrimal gland and distributed over the eye by motion of the eyelids. The tear film layer serves a number of biological functions including to lubricate and protect the corneal surface (epithelium). The tear film also includes several analytes that are present in blood, at concentrations that are related to their corresponding blood-analyte levels. Measuring the concentration of tear film analytes can therefore be used to diagnose and/or monitor various biological conditions.